Carbon nanotubes are a relatively new class of materials which, in their pure form are of great technological interest as mechanical reinforcing, electrically and thermally conducting additives for static protection. The present invention relates to a method of purification of the carbon nanotubes by a non-destructive and efficient method using a new type of polymer to extract them from the accompanying material without damage to their structure.
When friction and separation occurs between materials, a transfer of electrons from the atoms on the surface will take place. This process is referred to as triboelectric generation. The resulting imbalance of electrons is what is called an electrostatic charge. This electrostatic surface charge is either positive or negative depending on whether there is a deficiency or abundance of free electrons respectively. We refer to this charge state as static electricity because it tends to remain at rest or static unless acted upon by an outside force. The amount of charge generated through the process of friction and separation will be influenced by the extent of the contact, the materials involved, relative humidity, and the texture of the material. Static charges of up to 30,000 Volts (V) are not uncommon and can be generated by the simple act of walking across a floor; yet a discharge of only 10 V can destroy a class 1 electrostatic discharge (ESD) sensitive device. Static electricity is an essence invisible although we often see it effects and can feel and measure its presence or electrostatic field. Since it is created from an imbalance of electrons it is not in a natural or stable state. Material with an imbalance of electrons will, when possible, return to a balanced state. When this is done rapidly a zap or spark associated with rapid ESD occurs. We may feel the zap if the discharge that occurs is over 3000 V. Electrostatic discharges below that level are below the threshold of human sensation but are still lethal to electronic and associated semi-conductor devices.
One common misconception is that conductive materials do not generate charges. This is because the dissipation of static charges from grounded conductive material tends to be complete and rapid. Ungrounded conductors can generate and hold static charges.
The material the inhibits the generation of static charges from triboelectric generation is classified as antistatic. An antistatic material can be conductive, dissipative or even insulative. Only conductive or dissipative antistatic material should be used in ESD safe areas. Insulative material are more commonly understood to generate and hold a static charge. Since they are insulators they do not allow the charge to move or distribute throughout the object. Grounding is not an effective method of neutralising insulators. Static fields on insulators are not necessarily permanent either; they will eventually be neutralised by gradual recombination with free ions. Free ions are charged particles that occur naturally in air. They may be in the form of atoms, molecules, or group of molecules such as water droplets. As free ions pass near a charged object of the opposite polarity they are attached by the field and will gradually return the material to a state of balance. A charged object is surrounded by an electrostatic field. This field can also effect nearby objects by charge induction. Charge induction lets an electrostatically charged object charge other nearby objects without actually touching them; typically as far away as several feet.
In the processing of film materials of plastics, static material can cause materials to cling to each other causing product quality problems or production slow-down. In clean rooms, charged materials can hold static-laden dust, preventing these dust particles from being circulated and picked up by the filtration system. Microelectronics suffers a different type of quality problem due to static electricity. Electronic components are composed of micro miniature traces and structures of alternating layers that may be insulative, conductive or semi-conductive. Rapid ESD can cause damage to these underlying structures via the traces of the component. Unfortunately, ESD damage to electronic components is not as readily apparent as the effects of static electricity in other industries. This is because ESD damage is not generally visible as it occurs and may be latent or not show up in functional testing of electronic devices. ESD damage may lead to premature or intermittent failure. Estimates of the cost of ESD damage to electronic based equipment run as high as five billion dollars annually. The cost of ESD damage is not simply the cost of the components, but includes the cost of labour and may include all of the expenses associated with field repair. Another cost is that of lost business due to customer dissatisfaction.
Current methods used to combat static electricity include charge prevention, shielding and neutralisation. Charge prevention is accomplished by reducing the exposure to charge generating materials. Charge generation can be prevented through the elimination of unnecessary activities that create static charges, the removal of unnecessary materials that are known charge generators and the use of antistatic materials.
Antistatic materials are those materials that are shown to create minimal static charges generally less than 200 V, when exposed to friction and separation. Antistatic materials may be naturally low in charge generation properties or have been made or treated with an antistatic agent.
Carbon nanotubes are graphite sheets that are rolled up and closed at either end producing a closed tube of carbon atoms. Carbon nanotubes have an electronic character that ranges from semi-conducting to metallic. It is these unique electronic characteristics that confers on the carbon nanotubes their potential for use as antistatic agents.
Carbon nanotube production can be carried out using the Krxc3xa4tschmer generator where sublimation and recombination occur to form carbon nanotube soot from graphite rods in a plasma. To date, there have been problems purifying the carbon nanotubes from the soot. Methods that have previously been disclosed for purifying carbon nanotubes include purification by treatment with strong chemical oxidants, purification by burning of unpurified samples and purification using surfactants. One such method is described in No. U.S. Pat. No. 5,560,898. All of the previously disclosed methods have disadvantages. Chemical oxidants do remove the nanotubes from the impure soot but tend to break chemical bonds in the nanotubes, especially at the tips. Methods involving burning tend to produce better purity samples but the yields are very poor in the order of 1% to 2% yield of carbon nanotubes. Purification using surfactants is more efficient but still involves high power ultrasonic bath treatment which is again known to break nanotubes at their tips.
It is an object of the present invention to overcome these problems.
It is also an object of the invention to provide a method for purifying carbon nanotubes that is not destructive and is efficient and easy to reproduce.
According to the present invention there is provided a composition which includes nanotubes and an organic material. Preferably the organic material has a coiling structure. The term xe2x80x9ccoiling structurexe2x80x9d as used in this specification means a structure which facilitates the organic material wrapping about the nanotubes, that is capable of forming structures which wrap, coil, curve or bend around the nanotubes. The material may form strands and/or ropes for this purpose.
The term nanotube as used in this patent specification is taken to mean any nanostructure and related materials. The organic material may comprise one or more polymer (conjugated and non-conjugated), oligomer (conjugated and non-conjugated) and monomer (conjugated and non-conjugated) or combinations thereof. The nanotubes which are mixed with these can be in the form of carbon nanotubes, nanotubes of other materials such as vanadium pentoxide for example, nanostructures (regular and undefined), as well as derivatives of these which can be based on or contain, as an example, Silicon, Boron, Tin, nitrogen, compounds of vanadium and oxygen such as vanadium pentoxide, etc. The nanostructures can have dimensions from nanometers in length to millimeters in length, as well as nanometers in width to micrometers in width.
In a preferred embodiment of the invention the organic material is a polymer.
In a particularly preferred embodiment the polymer is poly(m-phenylene-co-2,5-dioctoxy-p-phenylenevinylene).
Various other coiling polymers, oligomers and aggregates can be used such as poly(dioctyl fluorene) or poly(sulphonic acid). Other polymers such as polyacetylene which can form strands and/or ropes could also be used. Further, DNA and all related coiling biological systems could be used.
According to another aspect of the invention, there is provided a process for purifying nanotube soot comprising the steps of:
adding nanotube soot to a solvent which includes a nanotube extracting material to form a solution;
mixing the solution to form a nanotube composite suspension and separate solid material;
allowing the separate solid material to settle;
removing the nanotube composite suspension.
The nanotube extracting material keeps the nanotubes in suspension while allowing the undesirable solid materials such as amorphous carbon to settle out. Preferably the nanotube extracting material is an organic material. Ideally the organic material has a coiling structure.
In a preferred embodiment the nanotube extracting material is one or more polymer, oligomer or monomer or combinations thereof.
In a preferred embodiment the nanotube extracting material is poly(m-phenylene-co-2,5-dioctoxy-p-phenylenevinylene).
Preferably the nanotube soot, nanotube extracting material and solvent are mixed in an optimized ratio dependent on the starting materials used. The solvent could be a liquid or gel. Any suitable solvent which can solubilise the nanotube extracting material can be used.
In another embodiment of the invention the solvent used is an organic solvent.
In another embodiment of the invention the organic solvent is an arene aromatic hydrocarbon.
Conveniently, the solution is mixed by sonication. However, any other suitable mixing method may be used.
In a preferred embodiment of the invention the solution is mixed in a low power ultrasonic bath for at least 20 minutes.
According to another aspect of the invention there is provided a process for making a nanotube and organic polymer suspension comprising the steps of mixing a solvent with an organic polymer to form a solution having a desired viscosity, said viscosity being sufficient to suspend nanotube containing material to the solution, and mixing the nanotube containing material through the solution to form a nanotube and organic polymer suspension.
In another aspect the invention provides a nanotube extracting polymer poly(m-phenylene-co-2,5-dioctoxy-p-phenylenevinylene, of the formula: 
The side groups can be changed if desired to change the helical structure. In some cases only one side group may be provided.
In a still further aspect the invention provides a method for preparing poly(m-phenylene-co-2,5-dioctoxy-p-phenylenevinylene)polymer comprising:
adding a phosphonate salt and an aldehyde to an ionizing solvent;
heating the mixture;
adding a potassium salt to the mixture;
allowing the mixture to react for a preset time period to form a polymer;
pouring the mixture into a solvent to enable the polymer to precipitate;
separating the polymer from the liquid;
drying the polymer; and
purifying the polymer.
In one embodiment of the invention the phosphonate salt, aldehyde and ionizing solvent are mixed in an optimised concentration ratio.
In one embodiment of the invention the phosphonate salt used is 1,4-bis(2,5-dioctoxy)benzyldiethyl-phosphonate.
In another embodiment of the invention the aldehyde used is terphthalaldehyde.
In another embodiment of the invention the ionising solvent is a formamide.
In one embodiment of the invention the polymer is prepared in an inert atmosphere.
In a preferred embodiment of the invention the inert atmosphere is an argon atmosphere.
In another embodiment of the invention the mixture is heated to between 70 and 90xc2x0 C.
In a preferred embodiment of the invention the mixture is heated to about 80xc2x0 C.
In another embodiment of the invention the potassium salt is potassium tert-butoxide.
In another embodiment of the invention the mixture is allowed to react for at least 3 hours.
In another embodiment of the invention the solvent used is water.
In another embodiment of the invention the polymer is separated from the liquid by centrifugation.
In another embodiment of the invention the polymer is dried under vacuum.
In another embodiment of the invention the polymer is purified by continuous extraction using an alcohol.
In another embodiment of the invention the polymer is purified by continuous extraction using a primary alcohol.
In another embodiment of the invention the alcohol is selected from the group including methanol, ethanol, propan-1-ol and phenylmethanol.